1. Field of the Invention
The present invention generally relates to a disk drive, and more particularly to a disk drive having a pivot embedded torque generating track follow actuator.
2. Description of the Related Art
Growth in areal density (bits/sq. inch) of a hard disk drive (HDD) is achieved through an increase in track density and bit density metrics. Technical advancement in electromechanical components and servo system architecture facilitates the increase in track density. Indirectly, an increase in track density requires a commensurate increase in crossover frequency of the track following servo transfer function. A 3.5xe2x80x3 HDD for server class applications reached a track density of 30 kTPI (tracks per inch) in year 2000, and the growth is expected to continue into the next decade.
Actuator resonance modes have become fundamental limiters in achieving higher servo crossover frequency required for high TPI design.
The sector servo system of a 3.5xe2x80x3 server class HDD with a 1 kHz openloop crossover frequency has been able to meet 30 kTPI (tracks per inch) track-following accuracy requirements. However, the growth of track density to higher than 30 kTPI has emerged as a major challenge to the actuator and servo system design.
Further, mechanical system resonance is a key limiter to higher bandwidth control. Use of microelectromechanical (MEMs) devices has been studied to increase actuator response characteristics. A major innovation in the actuator system design to increase the servo crossover frequency is desirable, but the storage industry needs cost-effective innovations in servo system design. A drastic change in the actuator system design does not retain the time-proven simple actuator system concepts. Thus, an alternative servo-mechanics approach is required to meet the high track density challenges. However, prior to the present invention, such an alternative, optimized approach has not been presented or developed.
For example, turning to FIGS. 1A-1B, a conventional rotary actuator assembly 110 of a disk drive has a single voice coil motor (VCM) 120. It produces a force about a pivoting point in order to generate a change in radial position of the read/write head.
FIG. 1A shows the conventional rotary actuator assembly 110 found in a HDD. The actuator (and actuator arm 115) is made to pivot (e.g., by a pivot bearing assembly 150) about an axis when the VCM 120 is activated. As shown the actuator assembly 110 further includes a pivot assembly body 130.
The pivot itself is composed of a pair of ball bearings 160A, 160B, as shown in FIG. 1B, which are assembled with an appropriate preload so that the pivoting function is made to be sufficiently free of rotational stiffness. The ball bearings 160A, 160B, along with an inner shaft 170, are fitted inside of a bearing sleeve or housing 180, with the pivot assembly body 130 being fitted over the pivot bearing assembly 150. Thus, the shaft and ball bearings support the entire actuator assembly 110. The linear radial stiffness of the bearings 160A, 160B is high enough to maintain the resonance of a rigid actuator to be around 10 kHz. In a xe2x80x9creal worldxe2x80x9d application, the radial stiffness of the pivot-bearing contributes to general reduction of the free-body vibration of the actuator assembly 110. Early recognition of pivot stiffness induced dynamics as a detractor and a solution to it can be found in commonly-assigned U.S. Pat. No. 5,267,110, incorporated herein by reference.
Recently several institutions have shown initiative in addressing the problem of finite radial stiffness (e.g., see K. Aruga, xe2x80x9cHigh-speed orthogonal power effect actuator for recording at over 10,000 TPI, IEEE Transactions on Magnetics, Vol. 32, No. 3, May 1996).
Turning now to FIGS. 2A-2B, there are several actuator resonance modes associated with a 3.5xe2x80x3 form factor HDD.
FIG. 2A shows a graph of magnitude with respect to frequency. That is, when a force (current) is applied to the actuator, the head is anticipated to move in a certain way (e.g., a certain frequency will result in the conventional actuator arm assembly).
The first important mode (e.g., resonance peak) that occurs around 7 kHz is understood to arise from bending of the actuator voice coil motor around its pivoting point. The coil bending resonance (CBR) is associated with a 180-degree phase change (e.g., see FIG. 2B which shows the phase as a function of frequency) and in certain configurations the magnitude/phase combination could produce an unstable condition of the track-follow servo. This bending mode characteristic also is sensitive to temperature, pivot parameters and other design parameters of a disk drive.
Conventional approaches of managing the presence of this mode have been to introduce a digital notch filter in series with the servo controller during a seek and track-follow mode. A notch filter reduces the negative effect of the peak gain that occurs due to the coil bending resonance (CBR). Because of the temperature-induced drift of the resonance frequency as well as the manufacturing variability encountered within a population of a product, the digital notch filters are designed to have wider than required attenuation bandwidth, thereby resulting in a corresponding phase loss in the crossover region of the servo loop. The loss of phase in turn limits the achievable crossover frequency of the track-follow servo system.
Another industry effort to tackle the CBR has been to include an active damping servo loop within the conventional positioning servo (e.g., see F. Huang, T. Semba, W. Imaino and F. Lee, xe2x80x9cActive Damping in HDD Actuator,xe2x80x9d Digests of APMRC2000,xe2x80x9d ISBN 0-7803-6254-3, November 2000, page MB6-01). This method, which is theoretically equivalent to that of an optimized digital notch filter, has been implemented in some server class HDDs.
A passive method to enhance the CBR resonance through structural modification is proposed in J. Heath, xe2x80x9cBoosting servo bandwidth,xe2x80x9d Digests of APMRC2000,xe2x80x9d ISBN 0-7803-6254-3, November 2000, page MP20-01. Briefly, suppressing the CBR by various methods has a time limited advantage, and it does not allow for progressive growth in servo crossover frequency required for next generation HDDs.
Thus, the impact of coil resonance in the track-follow servo transfer function must be minimized, and hence requires new innovations. The present actuator system with a single VCM is primarily optimized for seek operation. The track-follow performance is extracted from the same actuator structure as a secondary challenge. However, this constraint must be removed in order to achieve not only an optimum access but also a high track density settle-out and track follow performance. H. Yamura and K. Ono, xe2x80x9cNew H-infinity design for track-following,xe2x80x9d Digests of APMRC2000,xe2x80x9d ISBN 0-7803-6254-3, November 2000, page TA4-01 proposes a configuration in which the contribution of CBR is circumvented by a second actuator.
FIG. 3 shows a conventional disk torque generating actuator concept in which a generic torque producing VCM configuration for track-following operation is suggested (e.g., see the above-mentioned U.S. Pat. No. 5,267,110, incorporated herein by reference).
In FIG. 3, the torque generator 300 includes a main VCM 310, a pivot 320, a xe2x80x9cmini-VCMxe2x80x9d 325, a load-beam 330, and a head 340 which provides an input to a servo 350. The servo 350 also receives an input from a rotation velocity sensor/servo 360 coupled to the main VCM 310. The servo 350 provides outputs to the main VCM 310 and the mini-VCM 325 to move the head about the pivot.
It is noted that this system developed in that the previous conventional system employed only the main VCM. However, a problem arose in that, in applying a force to the arm (and thus the head) by the main VCM 310 (e.g., based on a signal from the servo), a clockwise torque should result, thereby moving the head in a clockwise direction.
However, because of the configuration of the previous conventional device, in applying the force (and moving the head) to create a clockwise torque, a force was also being produced along the pivot normal axis 370 of the actuator (e.g., upward). The normal axis 370 is orthogonal to the actuator long axis 380, as shown in FIG. 3. Due to the compliance of the pivot 320, a linear motion was also being produced in the normal axis 370 direction of the entire system, thereby moving the head in a direction opposite to where the head was desired to move (e.g., clockwise). Thus, the mini-VCM 325 was developed and provided to apply an opposite force to ensure the head was compensated for and moved in the desired clockwise direction.
However, with the provision of the mini-VCM 325 and trying to avoid the problems occurring with the compliance of the pivot, space problems have arose in the tight design space of the disk drive especially with disk drive platters provided over the actuator arms close to the pivot. Thus, these problems have made provision of a second coil unattractive in the conventional design.
Thus, the conventional systems have failed to produce an actuator structure that is capable of enhancing the track-follow performance without being constrained by the seek actuator design. However, realization of this concept in a product having disk platters (e.g., a tight, small-space environment) and other components sensitive to an electromagnetic field requires significant innovation.
Prior to the present invention, neither the advantages of such a concept have been recognized, let alone a practical development of such a concept even been undertaken. Indeed, there has been no system which has optimized the move/seek time for large displacements, compensated for the resonance features which appear as a result of the bearding/pivot compliance as well as the bending of the entire main-VCM structure (e.g., a relatively large structure), and yet simultaneously provided a compact system.
In view of the foregoing and other problems, drawbacks, and disadvantages of the conventional methods and structures, an object of the present invention is to provide an actuator structure (and method) which is capable of enhancing the track-follow performance without being constrained by the seek actuator design.
Another object is to realize such a concept in a product having disk platters and other components sensitive to an electromagnetic field.
Another object is to provide a method and system which provides compensation for a relatively low frequency resonance (e.g., having a peak around 7 kHz, as shown in FIGS. 2A-2B) and which, at the same time, optimizes the move/seek time for large displacements.
In a first aspect of the present invention, a disk drive system, includes an actuator system including a first voice coil motor (VCM), a second voice coil motor for enhancing dynamic resonance properties of the actuator system, and a single position error detecting mechanism commonly provided for the first and second voice coil motors.
In a second aspect, an actuator assembly for a disk drive system having a main voice coil motor (VCM), includes an actuator distributed to generate torque for track-following in addition to the main voice coil motor.
In a third aspect, a computer system, includes a disk drive system, and an actuator assembly for the disk drive system having a main voice coil motor (VCM), and an actuator distributed to generate torque for track-following in addition to the main voice coil motor.
In a fourth aspect, a pivot assembly for a disk drive system having a main voice coil motor (VCM), includes a pivot member, and an actuator embedded in the pivot member to generate torque for track-following in addition to the main voice coil motor.
In a fifth aspect, a spindle assembly for a disk drive system having a main voice coil motor (VCM), includes a spindle, and an actuator embedded in the spindle to generate torque for track-following in addition to the main voice coil motor.
In a sixth aspect of the present invention, a servo system assembly for a disk drive system, includes a first actuator, and a second actuator having a smaller form factor than the first actuator to generate torque for track-following in addition to the first actuator.
In a seventh aspect, a computer memory system, includes a disk drive system, and a servo system assembly for the disk drive system, the servo system including a first actuator, and a second actuator having a smaller form factor than the first actuator to generate torque for track-following in addition to the first actuator.
In an eighth aspect of the present invention, a server system, includes an actuator system including a first voice coil motor (VCM), and a second voice coil motor for enhancing dynamic resonance properties of the actuator system.
In a ninth aspect of the present invention, a method of generating torque for track following in a disk drive, includes providing an actuator system including a first voice coil motor (VCM), and distributing a second VCM in the actuator system for enhancing dynamic resonance properties of the actuator system and for generating torque for track-following in addition to the first VCM.
With the unique and unobvious aspects of the present invention, a system and method are provided in which an actuator structure enhances the track-follow performance without being constrained by the seek actuator design.
In this regard, the invention compensates for (e.g., negates) the effect of the low frequency resonance and simultaneously provides a practical drop-in solution (e.g., a retrofit onto existing systems with minimal disruption and redesign of the existing systems). That is, in situations where the conventional actuator is not enough to provide the required bandwidth (e.g., as track densities are increasing), the inventive actuator system can be xe2x80x9cdropped inxe2x80x9d in place of the conventional actuator, without demanding major changes in the way the rest of the drive components are developed.
Hence, of the options available, the conventional system operators need not xe2x80x9cgold platexe2x80x9d (e.g., fine-tune) the existing design of the systems, nor do they need to jump to an entirely new technology (e.g,. usage of MEMs, dual actuators, etc.). Instead, the system operators can use the invention as a xe2x80x9cdrop-inxe2x80x9d solution, thereby providing an integrated, proven system having great cost savings and minimal risk.
Additionally, all of the experiences of vendors of spindle motor design can be easily leveraged into making the pivot VCM.